Designing a small boat is not an easy task. The principal difficulty encountered comes from the weight of the crew members that could totalise almost half the entire mass of the boat it-self. The mobility of this weight compromised greatly boat lines and stability. So, imagine now, that you want to design a performing, easy towable, seaworthy family micro-cruiser. There, you will hit a real challenge. The boat should be as light as possible to be easily towable behind a small car but, in the mean time, it should be well ballasted to own the suitable stability when sailing on rough sea. It should be small enough to keep the ballast weight as low as possible and still be efficient and roomy to accommodate a small family aboard. To be fast, we need to maximize the flotation length and minimize wetted surface and drag of hull and appendages. Resolving this equation looks practically impossible to accomplish. The development of a new rig named “roto-duplex”, will help to solve, at least partially, this paradoxical assessment. But we have also to design and build a different type of craft especially dedicated to this kind of rig. That’s how Kasala project was born. Kasala is a double chine plywood hull of 5,5m /18’ length, a beam of 2,3 m/7’ 8’’ and a minimum draft of 0,315 m / 12”. With a STIX of nearly 19, it has a great stability and can carry 23 m² divided in two main sails. Despite this new kind of rig, several other features were investigate and included in this design:- High hull and appendages efficiency- Oscillating single rudder- Shock proof keel arrangement- Impact resistant lower plywood panels- Singular accommodation configuration- Ergonomic cockpit- Permanently mount crutches- Insubmersibility through the help of closed cell foam mattresses- Innovative main-sail hook- Powerful sculling oar- Efficient anchorKasala looks to me to be a perfect candidate to the Howard Fund grant. I will send by mail a complete documentation to the subject to the AYRS editor for future investigation. If some of you are interested, feel free to ask for more information.

Rather than words, a picture will show you what I call an oscillating rudder. But if you want to rename it, feel free! And here is the explanation I give in the presentation:“I decided to use a single rudder, even if the hull shape, from a plan view, looks like a wedge. With this kind of hull, when the boat reaches important heeling angles, a large surface of the rudder will pull out of the water. In response to this, KASALA’s rudder assembly will be able to swing from one side to the other on a circular sector bolted onto the transom. Handy ropes will operate its movements. By using a single rudder, we reduce weight, wetted surface and keep a lively tiller.Despite this, one can find a NACA 0010 profile compensate rudder that could be lifted completely out of the water when beaching.”But,sorry I don't know how to insert a pic! Alex.

One of Dashew's books has a picture of an Open 60 (or possibly a predecessor to that design rule) with such a rudder. I think it was called a tilting rudder there. I also saw it on Mudskipper, a steel cruising boat from Australia that came to Trondheim. There are pictures in one of the Junk rig Association Newsletters.

What advantage do you see compared to double rudders? A drawback I can see is having to shift the rudder during a tack or gybe, possibly quite quickly, while other events demand attention. A double rudder would seem to have the advantage that swinging up the windward rudder can wait until everything else is done, and lowering it again can happen before things get busy.

A boat is a choice of parameters and you have to keep in mind that Kasala is a laboratory. I'm the kind of helsman who like to sail with a long stick up to my shoulder and between two fingers! You can not do that with twin rudders. The tiller is often tough and not really pleasant to handle. Minimizing wetted surface and load is what drive me to go for a single rudder. But you are right, it could be questionable when tacking. But there again, before the tack the rudder can be swinged in central position and replaced proprely to leeward well after, when the boat sail on the new head.With the roto-duplex rig, the rudder is almost useless. Sooner, I will unveiled some highlights to this kind of rig and it will surely be more comprehensive.

Roto-duplex rig.Duplex rig also called twin cat rig or bi-plan rig consist of 2 masts lying side by side symmetrically at sheer line or mainly on each hull of a catamaran. This rig is rarely used on monohull due to the fact that, on close or beam reach, the windward sail shade the leeward one. With the roto-duplex as shown on pictures of Kasala model, the mast assembly can rotate and both sails can be exposed to “clear” or “fresh” wind. On closed reach for example, the leeward sail is moved forward. Before wind, or on broad reach, the two sails are wide open like butterfly wings. I find several benefit but also few drawback to this device.Benefit: - Large sail area with lower center of pressure- Only two identical main sail (no jib or spi)- Large possibility of sail reduction with only 2 reefing lines per sail- When furled the sails stay on the boom (no sails in cab)- Ease for jibbing one sail at a timeDrawback:- Tacking is more complex and time consuming- Restricted access to front deck To be proved: - Better efficiency. Eventually, with rotating small mast section, sail efficiency should be greater. Also, heading on beat should be better due to rig configuration and sails specifications.

Have you decided how to attach the masts to the circular track? I suppose that connection must cope with compression as well as upwards pull. And I think sail forces will always push the whole kaboodle to lee, so you must actively control the angle of the mast assembly to the wind?Rotation of the mast assembly trades off gap against stagger. You might be interested in two papers on "Gap and stagger effects on biplanes with end plates": https://arc.aiaa.org/doi/10.2514/6.2009-1085 and https://arc.aiaa.org/doi/abs/10.2514/6.2009-1086

Last edited by Robert Biegler on Tue Mar 14, 2017 7:47 pm, edited 1 time in total.

Well notice! Yes there is some pressure (or traction) on side mast base. This pressure could be estimated at 5000 N (1100 lbs) per side including an appropriate safety factor. In fact the rail is a hight quality grade aluminium "U" shape channel, perfectly bend and lay on his side on deck. Originaly,it was an UHMWPE shoe that slides in between the channel branch. This slide receive the pressure directly from the mast. But it's hard to operate at full pressure, and in the future, four Nylatron rollers will be added to each shoes and attached with a semi-elastic manner. You don't want any play or gap there but they still have to rotate freely. A tricky design, for sure! I know the papers you mentionned. Unfortunately they are not free accessible. So, I use more older references as the one of Prandlt or Munk. Also, a 2D CFD basic configuration shows that some sails arrangments could be relevant. But thank's.

AlexQ23 wrote:I'm the kind of helsman who like to sail with a long stick up to my shoulder and between two fingers! You can not do that with twin rudders.

Me, too, but back when I sailed a Hobie 14, that's how I had it set up, and with much of the lateral resistance being provided by the rudders, they saw some load. So fingertip control can be achieved with some twin rudders. What do you expect to prevent this on your boat?

On most catamaran the rudders are vertical. By contrast on beam transom monohull, the rudders are angling about 15 or 20°. This create some friction in mechanical linkage and increase the stiffness of the tiller. Diabolo rubber are often use to realize those attachment.With a duplex rig, like with a bi-plan airplane, the cell is more stable. Numerous studies confirm that and it's also confirm by experience. With Kasala, the model shows that the rudder is not really loaded and that the unit can stay straight on a steady course just with sails adjustment. We still have to prove that with the full size model.

Few words again on roto-duplex rig:It doesn’t seem that this rig arouse much enthusiasm. But we all know that sailors are conservative and are allergic to drastic change. Wind is a free energy and trying to improve efficiency of a sailing arrangement is for poor interest for a great majority of us. This is true! But we have to look forward and the benefit of such rig will, maybe one day, serve a most universal cause like freight ship propulsion, for example.When describing this kind of rig, I don’t want to use the word biplane. For most people, biplane is a reference to an early flying machine equipped with two pairs of wings. And then the comparison between monoplane and biplane is never far. This is not our case! Don’t forget that a sloop rig is also a biplane where the wings are much more in tandem configuration rather than standing side by side; but there is still a pair of sails! With the roto-duplex rig we will be able to trim the sails from “biplane” configuration to “tandem” one. It should be, somewhere in between this adjustment range, a practical arrangement that will deliver the maximum efficiency out the sails. The theory tells us that it could happen. But, a sailboat is not a flying machine; we are not operating at the same speed or with the same wing (or sail) shape or profile. So it’s quite difficult to extract a true prediction out of the different studies that are realized for airplanes. I really think that we have to try this on a real sailing model. That’s why the Kasala project is born. There is a time where experimentation should happen, most of the time to prove the soundness of an idea that will lead to new theory or a new vision of an existing theory. That’s how the science can grow. So I hope that why my project will be published, it will received some consideration.

HelloThe photos seem to suggest you have built the boat itself but the rig appears to be a model, I find your project very interesting but feel that the boat ought to be sailed with a conventional rig to establish its performance before moving on to the new rig.There are lots of novel features described in the hull which need to be proven before attempting the rig.FCB

I'm glad that you find the boat interesting! Unfortunately, the rig is integrally part of the project and another type of rig can not be fixed on those hull without affecting balance, stability and efficiency . But sails performances can be demonstrated with polar curves. Also, competing with other similar units like Aviator or Sailart18 can demonstrate rig performance. I'm sure that active members of AYRS could give me relevant advice to help me in this matter. Actually, the rig is completed too; I'm now working on booms that are also quiet unusual.

AlexQ23 wrote:Actually, the rig is completed too; I'm now working on booms that are also quiet unusual.

That was going to be my next question. I noticed the model has conventional booms and no apparent control over twist. There doesn't seem enough leverage for a vang, and a traveller might have problems with masts moving fore and aft. I would use wishbone booms. What solution have you come to, after a lot more thought?

Hello Robert, For sure a wishbone could surely do the work too! But... - Some are not so effective for fine tuning the sails. - The reefing is not as simple as it looks- With a rigid mast and a square top sails they are not particularly recommended - When the sail is furled you can’t leave it on post (or on the boom) So I prefer to go with a boom. The boom is a half 6 inches thin aluminum pipe, rigidified in the bottom with a full length rail. The main sheet traveller slides on this rail. There is a 20:1 cascade pulley boomvang and a gas spring that can keep the boom up. When reefing or furling the sail fall easily into the boom. This wide boom will also minimized the air mixing between intrados and extrados of the sail (pressure side and less pressure side). A small rope and a bungee cord keep the sheet traveller automatically at the same vertical position, no matter if the sail is going forward or backward.Keep in mind also that the mast rotates. Hope the 2D drawing will help you to understand how everything works!

AlexQ23 wrote:For sure a wishbone could surely do the work too! But... - Some are not so effective for fine tuning the sails.

Because you can't separate outhaul from leach tension? I think that could be done if you have a vang to regulate the vertical position of the end of the boom, and you attach the clew to a ring sliding on a short horizontal tube, with an outhaul attached to the ring. And the lever arm of that vang would be a lot more advantageous, so you wouldn't need to put so much compression load on the boom, and a lower point load where the vang attaches. Then again, Rob Denney uses simply a rigid boom, with bearings separated by a similar distance as your goose neck and vang, and it seems to work.

AlexQ23 wrote:- The reefing is not as simple as it looks

Probably not.

There is an alternative that has not, to my knowledge, been tried yet: use rigid, curved battens that rotate around the chord, like a Bierig camber spar, and sheet each batten like in a junk rig. In theory, that should give you good control over twist, low loads on the rig, and good reefing. And in theory, theory and practice are the same...

AlexQ23 wrote:- With a rigid mast and a square top sails they are not particularly recommended

I’m sorry, but I never use a junk rig and I don’t know anything about it. But, what I know is that I never see any winning racing sailboat equipped with such a rig. Bernard Moitessier sails with such rig when young but he never keeps it on her later craft.A square top sail is quite specific. The top of the sail should be able to twist in gust. It will act like a relief valve. When the top is flexing, the induced drag will collapse or at least diminish. It’s all a question of leech tension, flexion of sail battens and flexion of mast top. So it’s really not an easy process to design a sail for such consideration and it’s more than often a try an error process. So, if somebody can help me there, it will be well appreciate. You will find here enclosed what I propose to do with Kasala sails. Can we improve the profile or curvature or batten arrangement?

Select the right keel profile.A few weeks ago I complete installation of Kasala keel inside his trunk. Everything was going fine and the simple lifting system I adopt, allows a 10 years old boy to raise or lower the keel. It’s a 6:1 pulley system install on top of the cab, completed with an external small winch with a 10:1 handle. So it’s easy and simple to control the 330 lbs keel from cockpit. But it could be interesting to share with all of you, how I choose the right keel profile. As you surely known, we are looking to have a profile that can give us the maximum lift for the minimal drag. But the maximum lift at an angle of attack of around 4° is not a real problem, because all the profile we are looking for, deliver about the same amount of lift at this angle. So, limiting the drag is the issue. And the drag is directly proportional to the thickness of the profile. So, limiting the thickness of the profile is critical. With Kasala we can come with a 7.5 % of the cord, thanks to the two layers of carbon fibre that stiffen and encapsulate the hard wood core. We have also to keep in mind that we will have to cut the tip of the trailing edge and that, also, will add drag. But now, we will have to choose between several profiles. The most useful for us where design by NACA and more specifically, the four digit series, the 63 series, the 64 series or the 65 series. In a real interesting paper presented by David Vacanti, “Keel and rudder design”, easily found on the net, in fig 9, we can find at table 3, for each profile, the drag coefficient at maximum lift in regard to the foil thickness. And we can see that for about 7% thickness, the minimum drag occurs with the 63 series. So we adopt this profile for Kasala.

One point I would make with regard to keel section is that I think you need to consider the situation when the wind is strong, the boat is trying to go to windward so keel loading is high but speed through the water is very low due to the effect of rough water and windage. Might this require a different keel section to that which gives minimum drag under more favourable sailing conditions?John

Hi John! Thanks’ for your compliments. It’s an interesting topic you bring here. For sure, that when we select a keel profile we assume that it works in a laminar flow. But it’s true that it is rarely the case. However, I don’t think that the choice of a keel profile has something to do with the situation you describe.The lift of a keel is proportional to the lift coefficient, the speed squared and the area. A fin keel operates at high lift coefficient. But, when speed drop, as per your example, the keel can stall. It is a reason why the keel area is often more large than what it should do. Generally, we adopt a ratio between sail area and keel of 3% and 1% for the rudder. So you should have an appropriate keel area. But you should have also an appropriate Keel aspect ratio. A long keel for example with a low aspect ratio will never experience the same problem in this situation. You have to keep in mind that the lifting force of the centerboard, when the boat is close to the wind is directly opposed to the lifting force of the sails. The speed factor is also quite important. If you pay attention to sailing models, you will notice that the keel area looks larger than what it should be in real. It’s not rare to see a keel/sail area ratio of 20% on sailing models!So, when you boat is slow, proportionally, you need more keel area. The faster you go, the less “wing” you need. If we go back to the early days of airplanes, multiplans were the rule. As soon as we increase the speed, the lift will be provides by a single set of wings. And on a high speed missile, few winglets will do the job. The speed of our displacement boats will not vary much from one boat to another. With my 5,5 m I can reach 6 knots. A 9 meter will reach 7,5 knots. So for an increase in length of 63%, the increase in speed will be only 25%! Obviously, the keel is a complex matter and there again the choice of right parameters are critical.